28 Control of the Greenhouse White Fhj 



By taking continuous measurements of the growth of very vigorous 

 plants by means of a Farmer auxanometer it was found that when the 

 fumigation commences a distinct check in growth occurs when doses of 

 \-\ oz. per 1000 eft. are used, even if no subsequent lesions develop. 

 After a day or two a normal rate of growth is resumed. One of these 

 growth records is reproduced in Fig. 5. The plant from which this was 

 taken was young, about 2| ft. in height and of a moderately soft nature, 

 with the second truss setting. One pint of water was given daily. The 

 thread was tied close behind the growing point and the pointer was reset 

 at noon each day. The actual growth on six successive days was as 

 follows, the fumigation lasting on the third period from 9.0 p.m. to 



Fig. 5. Record of daily growth of tomato plant measured by i^armcr auxanometer, showing 

 check and recovery in growth after fumigation with liydrocyanic acid (j oz. cyanide 

 per 1000 eft., duration 9 hours). Instrument set to record double the growth. The 

 gas was introduced on the third day at the point marked by the arrow. 



(3.0 a.m.: (i) 1-25 mm., (2) -9 mm., (3) -8 mm., (4) -3 mm., (5) -35 mm., 

 (6) '95 mm. Very slight damage to the foliage followed the fumigation. 

 A similar check in the growth of the length of the leaves also occurs 

 though unaccompanied by any obvious damage. 



A natural assumption is that the gas causes damage by entering the 

 leaf through the stomata but this is not necessarily the case. In this 

 connection two experiments suggested by Prof. V. H. Blackman were 

 carried out. Two tomato plants were placed in a dark chamber 5 hours 

 before dusk, while four similar plants were kept in the light. At dusk 

 all the plants were placed close together and fumigated with \ oz. of 

 cyanide per 1000 eft., duration ^\ hours, temperature 69-60°, relative 

 humidity 97 per cent. The experiment was repeated with a plant kept 



